Effects of nano size mischmetal and its oxide on improving the hydrogen sorption behaviour of MgH2

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Effects of nano size mischmetal and its oxide onimproving the hydrogen sorption behaviour ofMgH2

T. Sadhasivama,b, M. Sterlin Leo Hudson a,c, Sunita K. Pandey a,Ashish Bhatnagar a, Milind K. Singh a, K. Gurunathan b, O.N. Srivastava a,*aHydrogen Storage Mission Mode MNRE Project Unit, Hydrogen Energy Centre, Department of Physics, Banaras

Hindu University, Varanasi 221005, IndiabDepartment of Nanoscience and Technology, Alagappa University, Karaikudi 630003, IndiacDepartment of Physics, Central University of Tamil Nadu, Thiruvarur 61004, India

a r t i c l e i n f o

Article history:

Received 6 October 2012

Received in revised form

5 April 2013

Accepted 8 April 2013

Available online 7 May 2013

Keywords:

Magnesium hydride

Nanoparticles

Ball-milling

Hydrogen storage

Synergistic effect

Activation energy

* Corresponding author. Tel.: þ91 0542 23684E-mail addresses: [email protected], h

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.04.0

a b s t r a c t

Thispaper reports thecatalytic effects ofmischmetal (Mm)andmischmetal oxide (Mm-oxide)

on improving the dehydrogenation and rehydrogenation behaviour of magnesium hydride

(MgH2). It has been found that 5 wt.% is the optimum catalyst (Mm/Mm-oxide) concentration

for MgH2. The Mm and Mm-oxide catalyzed MgH2 exhibits hydrogen desorption at signifi-

cantly lower temperature and also fast rehydrogenation kinetics compared to ball-milled

MgH2 under identical conditions of temperature and pressure. The onset desorption tem-

perature for MgH2 catalyzed with Mm and Mm-oxide are 323 �C and 305 �C, respectively.

Whereas the onset desorption temperature for the ball-milledMgH2 is 381 �C. Thus, there is a

lowering of onset desorption temperature by 58 �C for Mm and by 76 �C for Mm-oxide. The

dehydrogenation activation energy of Mm-oxide catalyzed MgH2 is 66 kJ/mol. It is 35 kJ/mol

lower than ball-milled MgH2. Additionally, the Mm-oxide catalyzed dehydrogenated Mg ex-

hibits faster rehydrogenation kinetics. It has been noticed that in the first 10 min, the Mm-

oxide catalyzed Mg (dehydrogenated MgH2) has absorbed up to 4.75 wt.% H2 at 315 �C under

15atmospherehydrogenpressure.Theactivationenergydetermined for the rehydrogenation

ofMm-oxide catalyzedMg isw62 kJ/mol, whereas that for the ball-milledMg alone isw91 kJ/

mol. Thus, there is a decrease in absorptionactivationenergybyw29kJ/mol for theMm-oxide

catalyzedMg. In addition,Mm-oxide is thenativemixture of CeO2 andLa2O3whichmakes the

duoabetter catalyst thanCeO2,which is knowntobeaneffective catalyst forMgH2. This takes

place due to the synergistic effect of CeO2 and La2O3. It can thus be said that Mm-oxide is an

effective catalyst for improving the hydrogen sorption behaviour of MgH2.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction two major issues associated with fossil fuels, the urban air

Produced from water, hydrogen burns back to water on hot

combustion (IC engine) or cold combustion (fuel cell).

Hydrogen is thus completely renewable, and it takes care of

68; fax: þ91 0542 [email protected] (O.2013, Hydrogen Energy P40

pollution and climate-change effect [1]. These properties

make hydrogen as a green fuel [2]. For harnessing hydrogen all

the major components namely production, distribution,

storage and applications need to be addressed. However, at

.N. Srivastava).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 27354

present it is realized that hydrogen storage is the most crucial

component which cuts across production, distribution and

applications. It is generally believed that solid-state hydrogen

storage materials that are sponge-like solid capable of

absorbing (and then desorbing) significant amount of

hydrogen forms potential storage candidates. These are

metal/intermetallic solids like LaNi5, FeTi, ZrFe2, etc. The

other variety is the elemental hydrides such as AlH3, MgH2

and complex (built-in) hydrides typified by NaAlH4 and other

related compounds. Unlike, intermetallic hydrides, for built-in

hydrides, hydrogen is first desorbed and then the dehydro-

genated material is hydrogenated to regenerate the initial

hydride.

At present, significant research activity is focused onMgH2.

This is so since it possesses a comparatively high hydrogen

storage capacity both weight wise (7.6 wt.% H2) and volume

wise (110 kg/m3) [3,4]. This is one of the closest to the required

US-DOE targets of 5.5wt.% and 40 kg/m3 [5]. Of course, the DOE

target corresponds to system. Therefore, material storage

capacities will have to be much higher. However, attempts

should also be made to make hydride container tanks with as

small weight as possible. Some efforts in this direction are

being made by us based on fabrication on carbon tubes con-

sisting of carbon nanotubes which are known to be stronger

than steel [6]. Another characteristic which makes MgH2 an

attractive candidate for hydrogen storage is that unlike other

built-in hydrides like NaAlH4, it exhibits nearly complete

reversibility. However, to make MgH2 as a viable hydrogen

storage material, two of the disadvantages associated with

MgH2 have to be overcome. One of the disadvantage is that, it

is highly stable and needs high temperature for destabiliza-

tion to release hydrogen. The other difficulty relating to MgH2

is very sluggish sorption kinetics [7].

There have been several studies in the last few years to find

effective ways to address the above said issues associated

withMgH2. Themost effectiveways are particle size reduction

[4], admixing metal [8e10], intermetallic storage alloy [11,12],

halides [13e16] and other suitable materials [17e26] to

destabilize MgH2. It is generally believed that these additives

work as a catalyst (lower the desorption temperature and

improve the hydrogen desorption kinetics). These also assist

in the opposite direction to regenerate MgH2 after desorption

(reversibility). One category of metals and their oxides which

have been most widely used as an effective catalyst for MgH2

are transition metals and their oxides [8,10,17e20,22]. The

present work describes the catalytic activity of mischmetal

(mixture of rare-earth metals, dominantly Ce and La) and its

oxide in lowering the sorption (desorption and absorption)

temperature and improving the sorption kinetics of MgH2. We

have employed Ce richmischmetal (Ce: 62.5 at.%, La: 30.0 at.%

and the negligible amounts of Pr and Nd) and its corre-

sponding oxide (Mm-oxide) as a catalyst for MgH2. The ad-

vantages of mischmetal (Mm) and its oxide as compared to

other transition metals and their oxides is that, Mm is cost-

effective and Mm-oxide gets readily prepared through low

temperature (w200 �C) oxidation of Mm [27]. In addition, the

dehydrogenation and rehydrogenation characteristics of

MgH2 employing Mm-oxide catalyst is better than some of the

known oxide catalysts, for example Al2O3, TiO2, Cr2O3, CeO2,

etc. [17,18,20,22,26].

2. Experimental section

2.1. Sample preparation

Commercial MgH2 (Alfa Aesar, 98%) and fine granules of Mm

obtained through scuffing bulk pieces of Mm (Leico, 99%) were

used in the present study. Commercially obtained bulk piece

ofMmpreserved inmineral oil was scuffed into smaller pieces

of millimetre size using a stainless steel cutting saw under

inert atmosphere. The Mm granules of millimetre size thus

obtained were ball-milled to turn these granules into fine

powder form. Mm-oxide was prepared by oxidizing the fine

Mm powder in a tube furnace at 200 �C for 60 min.

2.2. Ball-milling

Ball-milling was carried out using Retsch planetary miller (PM

400) and locally fabricated stainless steel high pressuremilling

vial (capable of retaining up to 100 atm). 2.5 g of MgH2 together

with 5 wt.% catalyst (Mm/Mm-oxide) was loaded in the 250cc

milling vial and ball-milled under hydrogen atmosphere

(w15 atm) for 25 h at an operating speed of 150 rpm. The ball to

powder ratio was kept at 40:1. Ball-milling under hydrogen

atmosphere prevents the formation of MgO and moisture

contamination. Handling of samples was done under the inert

atmosphere in an argon filled glove box (mBRAun, MB10

Compact) with H2O and O2 <1 ppm.

2.3. Hydrogen sorption analysis

Dehydrogenation behaviour and reabsorption kinetics of the

samples were analyzed through temperature programmed

desorption (TPD) andpressure composition isotherm (PCI) using

an automated four channel Sieverts type apparatus (Advanced

Materials Corporation, USA). About 150 mg of the sample was

loaded in the sample chamber and then inserted into the pro-

grammable solid tube furnace (Thermcraft). The TPD analysis

was carried out at the initial pressure of w10�2 Torr under

continuousheating rate of 5 �C/minwith an accuracy of�0.1 �C/min. Isothermal dehydrogenation kinetics of MgH2 samples

weredeterminedat 325 �C,337 �C,350 �Cand363 �C, respectivelyunder1atmH2pressure.Thiswascarriedoutby initiallysoaking

thesampleunder30atmH2pressureat roomtemperature.Then

the temperaturewas raisedgradually fromroomtemperature to

the desired isothermal temperature. Kinetics at 1 atmwas then

determinedby releasing thepressure from30atmto1atmusing

the release mode available in the AMC Sieverts type apparatus.

The release mode maintains the pressure constant during the

isothermal condition and monitors the amount of hydrogen

released with respect to time. Rehydrogenation kinetics of the

samplewasmeasuredbyusing the soakmodeavailable in thePCI

measurement system. The soak mode permits the desired pres-

sure into the sample chamber and periodically collects data

corresponding to the amountof gasabsorbed/adsorbed vs. time.

2.4. Characterization

Structural characterizations of the samples were carried out

through X-ray diffraction (XRD) using X’Pert PRO (PANalytical)

Fig. 1 e TEM micrograph of as synthesized Mm-oxide

nanoparticles.

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X-ray diffractometer equipped with a graphite mono-

chromator employing CuKa radiation (l ¼ 1.541 A) at room

temperature. The XRD sample holder was sealed by a fine

layer of parafilm (Pechiney plastic packing) to prevent the

sample from oxygen and moisture contamination. Particle

sizes of the ball-milled materials were characterized by using

the transmission electron microscope (TECHNAI, 20G2).

The surface morphology of pristine and ball-milled materials

were carried out using scanning electron microscope

(Quanta 200).

Fig. 2 e X-ray diffractogram of (a) pristine MgH2, (b) ball-milled M

(e) dehydrogenated Mm-oxide catalyzed MgH2 (Mg) and (e) rehy

3. Results and discussions

3.1. Structural/micro structural analysis

During oxidation of Mmparticles at 200 �C, it has been noticed

that there is a colour change of sample from dark brown to

greenish yellow. A representative optical image of ball-milled

Mm particles and Mm-oxide particles are shown in the sup-

plementary figure, Fig. S1(a) & (b), respectively. The X-ray

diffractogram of Mm particles reveals the dominant presence

of La and Ce, whereas in Mm-oxide, La2O3 and CeO2 formed

themajority phases (refer Supplementary Fig. S2(a) & (b)). TEM

analysis of Mm-oxide reveals that the particle sizes are of

<20 nm. A representative TEM micrograph is shown in Fig. 1.

The inset of Fig. 1 shows the selected area electron diffraction

pattern of Mm-oxide (native mixture of CeO2 and La2O3). This

pattern could be indexed successfully based on known lattice

structure of La2O3 (hexagonal with a ¼ b ¼ 4.060 A, c ¼ 6.410 A)

and CeO2 (fcc with a¼ b¼ c¼ 5.411 A). Thus, the oxidized form

of Mm corresponds to Mm-oxide. It also shows that Mm-oxide

particles are of nano-form. Similar TEM characterization of

the ball-milled Mm catalyst particles showed that their sizes

are also in the nano range.

Fig. 2 shows the X-ray diffractogram of (a) pristine MgH2,

(b) ball-milled MgH2, (c) Mm catalyzed MgH2 and (d) Mm-

oxide catalyzed MgH2. A noticeable feature observed in X-

ray diffractogram is that the peaks corresponding to Mm-

oxide catalyzed MgH2 are comparatively broader than that

observed for Mm catalyzed and ball-milled MgH2 under

identical conditions. Since the Mm-oxide has the higher

gH2, (c) Mm catalyzed MgH2, (d) Mm-oxide catalyzed MgH2,

drogenated Mm-oxide catalyzed MgH2.

Fig. 3 e SEM micrographs of (a) pristine MgH2, (b) ball-milled MgH2, (c) ball-milled MgH2 D 5 wt.% Mm and (d) ball-milled

MgH2 D 5 wt.% Mm-oxide.

Fig. 4 e TEM micrograph of Mm-oxide catalyzed MgH2

(inset image shows the SAED pattern).

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degree of hardness than MgH2, the most likely reason is that

MgH2 gets pulverized by Mm-oxide during ball-milling. This

helps to form homogeneous distribution of catalysts and fine

particles of MgH2. Fig. 2(e) shows the XRD of dehydrogenated

Mm-oxide catalyzed Mg. As it can be seen that the peaks of

Mg and catalyst are discernible, indicating a complete dehy-

drogenation of MgH2. Fig. 2(f) shows the XRD pattern of

rehydrogenated Mm-oxide catalyzed MgH2. It has been

observed that upon rehydrogenation (315 �C and 15 atm H2

pressure), the crystallite size has increased due to clustering

and segregation of macroscopic phases. Furthermore, a

fraction of Mg remains un-reacted upon rehydrogenation.

This clearly suggests agglomeration of Mg after dehydroge-

nation making the rehydrogenation difficult. The approxi-

mate particle size of MgH2 samples determined by using

Scherrer formula are, 30.65 nm, 21.54 nm, 18.90 nm and

14.19 nm for the pristine, ball-milled, Mm and Mm-oxide

catalyzed MgH2, respectively. It has been found that with

the addition of Mm-oxide, the particle size of MgH2 gets

decreased to w15 nm.

Representative scanning electron micrograph (secondary

electron image) of pristine MgH2, ball-milled MgH2, Mm cata-

lyzedMgH2 andMm-oxide catalyzedMgH2 are shown in Fig. 3.

As it can be seen in Fig. 3(a), the particle size of pristine MgH2

is in the range ofw50 to 100 mm. After ball-milling, the particle

size of MgH2 gets decreased to w2 to 5 mm (Fig. 3(b)). As it can

be seen in Fig. 3(d), the particle sizes of Mm-oxide catalyzed

MgH2 are in the range of <1 mm. Moreover, the distribution of

particles here, unlike in Fig. 3(a, b and c) are nearly uniform. It

is expected that upon the ball-milling MgH2 gets pulverized by

the Mm-oxide which has higher hardness than MgH2.

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Fig. 4 brings out the typical transmission electron micro-

graph (TEM) of ball-milled Mm-oxide catalyzed MgH2. The

inset image shows the selected area electron diffraction

(SAED) pattern. The bright spots in this pattern could be

indexed based on the lattice structure of MgH2 (tetragonal,

primitive with a ¼ b ¼ 4.517 A, c ¼ 3.020 A). On the other hand,

the continuous ring was explicable based on the lattice

structure of CeO2 (fccwith a¼ b¼ c¼ 5.411 A). As it can be seen

in the TEM micrograph, the Mm-oxide nanoparticles typically

of the sizes w15 to 20 nm are embedded in the large MgH2

(w400 nm) particle.

3.2. Hydrogen desorption/absorption studies

It may be pointed out that in the present studies, the con-

centration of Mm and Mm-oxide catalysts on MgH2 have been

varied from 2 to 8 wt.% (2, 5 and 8 wt.%). However, the opti-

mum results in regard to catalytic effect leading to the

improvement in sorption behaviour of MgH2 was obtained for

Fig. 5 e (a) TPD curves of pristine, ball-milled (BM), Mm and

Mm-oxide catalyzedMgH2 at the heating rate of 5 �C/min, (b)

TPDrateprofilecharacteristic curvesofMgH2at5 �C/minwith

respect to peak desorption temperature (derived from (a)).

5 wt.% catalyst. Therefore, we have used this catalyst con-

centration for all our studies reported in this communication.

Fig. 5(a) shows the temperature programmed desorption

(TPD) curves of pristine MgH2, ball-milled MgH2, Mm andMm-

oxide catalyzed MgH2 at the heating rate of 5 �C/min. The

degree of improvement in dehydogenation for MgH2 priority

wise is Mm-oxide catalyzed MgH2 > Mm catalyzed

MgH2 > ball-milled MgH2 > pristine MgH2. Thus for the heat-

ing rate of 5 �C/min, the onset desorption temperature ofMgH2

(Fig. 5(a)) has been lowered from 410 �C to 305 �C upon ball-

milling MgH2 with 5 wt.% Mm-oxide.

As it can be seen in Fig. 5(a), the hydrogen desorption from

pristineMgH2 begins at 410 �C and that for ball-milledMgH2 at

381 �C. However, Mm and its oxide facilitate hydrogen

desorption fromMgH2 at lower temperatures than that of ball-

milled MgH2. Thus for 5 wt.% Mm and Mm-oxide catalyzed

MgH2, the hydrogen desorption starts at 323 �C and 305 �C,respectively. Therefore, the onset hydrogen desorption tem-

perature of pristine MgH2 gets lowered significantly by 105 �Cupon admixing 5 wt.% Mm-oxide. This is lower by 76 �C when

compared with ball-milled MgH2. This decrease is found to be

higher than that obtained from Mm catalyzed MgH2, where

the onset desorption temperature is 323 �C.Fig. 5(b) shows the TPD profile characteristic curves at the

heating rate of 5 �C/min in terms of peak desorption temper-

ature. As it is clear from Fig. 5(b), the Mm and Mm-oxide

catalyzed MgH2 shows hydrogen desorption at lower tem-

peratures. However, Mm-oxide catalyzed MgH2 is found to be

superior than Mm catalyzed MgH2. The Mm-oxide catalyzed

MgH2 exhibits hydrogen desorption in two steps. The first step

is observed at 310 �C and the second step at 351 �C. These two

desorption temperatures presumably correspond to the cat-

alytic effects of CeO2 and La2O3, the two constituents of Mm-

oxide. The desorption temperature of w310 �C is one of the

lowest obtained so far for MgH2 and its catalyzed versions.

Rehydrogenation kinetics determined for Mg samples

(ball-milled, Mm and Mm-oxide catalyzed Mg) is shown in

Fig. 6. Hydrogen absorption kinetics was measured at 315 �C

Fig. 6 e Rehydrogenation kinetics of dehydrogenated Mg

catalyzed with Mm and Mm-oxide.

Fig. 7 e Dehydrogenation kinetics of (a) ball-milled MgH2, (b) 5 wt.% Mm catalyzed MgH2, (c) 5 wt.% Mm-oxide catalyzed

MgH2 at different temperatures under 1 atm H2 and (d). Arrhenius plot (ln(k) vs 1000/T ) of MgH2 samples.

Table 1 e Comparison of dehydrogenation and rehydrogenation activation energy values from literature with presentinvestigations.

Materials Dehydrogenationactivation

energy, Ea (kJ/mol)

Rehydrogenationactivation

energy, Ea (kJ/mol)

Ref.

MgH2 þ 50 wt.% Al2O3 80 * [31]

MgH2 þ 30 wt.% LaNi5 80 * [32]

Mg þ 40 wt.% ZrFe1.4Cr0.6 <94 * [33]

Mg þ 4 wt.% Nano Fe * 56 � 3 [37]

MgH2 þ 5 wt.% Nano Ni * 60 [38]

MgH2 þ 2 wt.% CNS * 66 [25]

MgH2 þ 7 wt.% ZrF4 82 * [16]

MgH2 þ 7 wt.% NbF5 70 *

MgH2 þ 7 wt.% TaT5 95 *

MgH2 þ 7 wt.% TiCl3 79 *

MgH2 þ 7 wt.% VCl3 96 *

MgH2 þ 1 mol% Nb2O5 71 � 3 * [19]

MgH2 þ 0.2 mole% Nb2O5 62 * [34]

MgH2 þ 1 mol.% BaRuO3 90 * [21]

MgH2 þ 20 wt.% TiO2 72 � 3 * [35]

MgH2 þ 5 wt.% SWNT 96 * [36]

MgH2 (ball milled) 101 91 Present study

MgH2 þ 5 wt.% Mm 81 70

MgH2 þ 5 wt.% Mm-Oxide 66 62

* Implies that results on these are not available.

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Fig. 8 e Rehydrogenation kinetic curves determined at

different temperature (a). Ball-milled Mg, (b) 5 wt.% Mm

catalyzed Mg and (c) 5 wt.% Mm-oxide catalyzed Mg.

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and 15 atm hydrogen pressure. A noticeable feature in Fig. 6 is

that the Mm-oxide catalyzed material shows significant

improvement in the rehydrogenation behaviour. The Mm-

oxide catalyzed dehydrogenated Mg reabsorbs 4.75 wt.% H2

within 10 min (5.5 wt.% H2 within 40 min). Whereas, Mm

catalyzed and ball-milled Mg under identical conditions

reabsorb only 4.32 and 3.25 wt.% H2 respectively. The wt.% H2

described here is with respect to the total weight of the ma-

terial (MgH2 together with the catalyst).

3.3. Determination of activation energy

To determine the effect of the catalysts Mm andMm-oxide on

the sorption characteristics of MgH2, we have evaluated

the activation energy (Ea) for dehydrogenation and rehy-

drogenation of Mm-oxide catalyzed MgH2, Mm catalyzed

MgH2 and ball-milled MgH2 by using JohnsoneMehleAvrami

[28] coupled with Arrhenius equation. The rate constant k

has been determined from the kinetic model for nucleation

and crystal growth in solids formulated by John-

soneMehleAvrami. This is given in the following equation:

[�ln(1 � a)]1/n ¼ kt (1)

where, ‘a’ is the extent of the reaction which can be iden-

tifiedwith a normalized hydrogenwt.% (range: from 0 to 1), t is

the time, k and n are constants (at constant temperature).

Arrhenius equation representing the dependence of rate

constant ‘k’ of the reaction, gas constant R and the absolute

temperature T is given by

k ¼ Aexp(eEa/RT ) (2)

Logarithm of Eq. (2) corresponds to a straight line and Ea can

be determined from the slope.

3.3.1. Dehydrogenation kineticsFig. 7 shows the isothermal dehydrogenation (desorption) ki-

netics determined at different temperatures (325 �C, 337 �C,350 �C and 363 �C) under 1 atm H2 pressure for (a) ball-milled

MgH2, (b) Mm catalyzed MgH2 and (c) Mm-oxide catalyzed

MgH2. It has been noticed that at 325 �C under 1 atmhydrogen,

the Mm-oxide catalyzed MgH2 has desorbed 5 wt.% H2 within

10 min, but Mm catalyzed and ball-milled MgH2 desorbed

5wt.%H2 in 20min and 50min, respectively. It may be pointed

out that the increase in desorption kinetics is only due to the

effect of catalyst (which remains intact and does not react

with MgH2) and not due to any additives to MgH2. A repre-

sentative figure comparing the kinetics of ball-milled MgH2

and MgH2 catalyzed with Mm and Mm-oxide is given in sup-

plementary figure (Fig. S3). It clearly suggests that the kinetics

of Mm-oxide catalyzed MgH2 is improved significantly than

both ball-milled and Mm catalyzed MgH2. Among various

metal oxides (Cr2O3, TiO2, Fe3O4, Fe2O3, In2O3 and ZnO) cata-

lyzed MgH2, Polanski et al. [17] reported that Cr2O3 and TiO2

catalyzed MgH2 exhibit superior dehydrogenation kinetics at

325 �C under 1 atm H2 pressure. In the present study, it has

been observed that the dehydrogenation kinetics of Mm-oxide

catalyzed MgH2 is similar to that reported for MgH2 catalyzed

Table 2 e Summary of rehydrogenation kinetic data obtained in the present investigation.

Temperature(�C)

Mm-oxide catalyzed Mg (wt.% of H) Mm catalyzed Mg (wt.% of H) Ball-milled Mg (wt.% of H)

10 min 20 min 40 min 120 min 10 min 20 min 40 min 120 min 10 min 20 min 40 min 120 min

315 4.75 5.18 5.43 5.57 4.32 4.50 4.62 4.70 3.25 3.84 4.15 4.38

292 4.12 4.6 4.8 4.91 4.08 4.19 4.26 4.46 3.09 3.62 3.90 4.25

255 2.8 3.54 4.2 4.88 3.10 3.42 3.81 4.20 2.20 3.00 3.61 4.10

236 1.65 2.48 3.36 4.09 1.60 2.20 2.66 3.23 1.27 1.73 2.16 2.73

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with Cr2O3 and TiO2. It may be pointed out, Mm-oxide cata-

lyzed MgH2 exhibit better dehydrogenation kinetics than

some other known catalysts [21,29,30].

JohnsoneMehleAvrami and Arrhenius equations were

used to determine the dehydrogenation activation energy for

catalyzed MgH2. As discussed earlier the activation energy for

dehydrogenation of MgH2 has been calculated from the slope

of Arrhenius plot (Fig. 7(d)). It has been determined that the

dehydrogenation activation energy (Ea) for ball-milled MgH2

and MgH2 catalyzed with Mm andMm-oxide arew101 kJ/mol,

w81 kJ/mol and w66 kJ/mol, respectively. Table 1 compares

the activation energy value reported for catalyzed MgH2.

The activation energy corresponding to Mm-oxide catalyzed

MgH2 is comparable with that of MgH2 catalyzed with

Nb2O5, which is thought to be a very effective catalyst for

MgH2.

3.3.2. Rehydrogenation kineticsIn order to determine activation energy for rehydrogenation of

Mg, we have evaluated the kinetics of dehydrogenated sam-

ples at different temperatures (315 �C, 292 �C, 255 �C and

236 �C) under 15 atm hydrogen pressure. Fig. 8 shows the

rehydrogenation (absorption) kinetic curves of dehydro-

genated (a) ball-milled Mg (b) Mm catalyzed Mg and (c) Mm-

oxide catalyzed Mg. Higher reabsorption capacity was ach-

ieved at 315 �C. However, as the temperature decreases, the

rehydrogenation capacity gets lower and the kinetics become

slower. The hydrogen reabsorption kinetic data obtained at

different temperatures for the ball-milled, Mm and Mm-oxide

catalyzed Mg are summarized in Table 2.

The JohnsoneMehleAvrami equation has been used to plot

a graph between [�ln(1 � a)] and ln(t) in which isothermal

experimental values are linear (Fig. 9(a)). The logarithmic

Fig. 9 e (a) Avrami plot (ln(Lln(1 L a)) vs. ln(t)) and (b). Arrheni

transforms of the equation have been calculated from hy-

drogenation curves as shown in Fig. 8(c). The John-

soneMehleAvrami plot of ln(�ln(1 � a)) vs ln (t) gives a

straight line and from the slope of that line, the rate constant

(k) can be determined. Using the Arrhenius equation, the

activation energy for the rehydrogenation of Mg has been

determined from the slope of Arrhenius plot (Fig. 9(b)).

The representative JohnsoneMehleAvrami plot and

the respective Arrhenius plot for Mm-oxide catalyzed Mg

is shown in Fig. 9(a) & (b). The corresponding John-

soneMehleAvrami and Arrhenius plots for ball-milledMg and

Mm catalyzedMg are given in supplementary figures, Fig. S4(a

& b) and Fig. S5(a & b). It has been determined that the

hydrogen absorption activation energy (Ea) for Mm and Mm-

oxide catalyzed Mg are w70 kJ/mol and w62 kJ/mol, respec-

tively. For ball-milled Mg it is w91 kJ/mol. Table 1 gives the

comparative activation energy values of catalyzed MgH2 re-

ported in literature. The absorption activation energy deter-

mined for Mm-oxide catalyzed Mg is 62 kJ/mol. This is very

much comparable to that reported for Mg catalyzed with nano

Fe (56 � 3 kJ/mol) [37] and nano Ni (60 kJ/mol) [38]. Our earlier

studies reveal that the activation energy corresponding to the

hydrogenation kinetics of effective carbon nanostructures

catalyzed Mg is 66 kJ/mol [25]. Thus, the present study unveil

that the hydrogenation activation energy determined for Mm-

oxide catalyzed Mg is lower than that of Mm and carbon

nanostructure catalyzed Mg.

3.4. Synergistic effect arising due to the presence ofLa2O3 together with CeO2

The improvement in sorption behaviour of catalyzed MgH2

(i.e, ball-milling together with catalyst) will increase the

us plot (ln(k) vs 1000/T ) of 5 wt.% Mm-oxide catalyzed Mg.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 7 3 5 3e7 3 6 2 7361

surface area and defect densities in MgH2, creating an easy

path for hydrogen atoms to diffuse and hence enhance the

hydrogen sorption kinetics. Improvement in desorption

behaviour of MgH2 might be due to the electronic exchange

reaction (reduction/oxidation) between the catalyst nano-

particles and MgH2 which are responsible for weakening the

bond between Mg and H. It is known that the reducible oxides

such as TiO2 and CeO2 contain cations of multiple valance,

which can easily undergo reduction to become TiO2�x and

CeO2�x. These oxides can undergo reduction and oxidation,

and hence they are known to be effective catalysts [39,40]. In

the present case, during sorption process, the CeO2 of native

Mm-oxide catalyst is expected to undergo reduction and

oxidation leading to the electronic exchange between the

catalysts and MgH2. Additionally, there is yet another oxide,

i.e, La2O3 which is present in Mm-oxide. This oxide is not

expected to get easily reduced. However, the presence of La2O3

can enhance the catalytic effect of CeO2. This is so, since La2O3

[41,42] has hardness of 8 Mohs as against 6 Mohs for CeO2 [43]

and 4 Mohs for MgH2 [44,45]. Thus, La2O3 will work like a

dispersing and cracking agent for MgH2. This will imply that

MgH2 would obtain smaller particle size in lower milling time

than that required in the absence of La2O3. The lowering of

desorption temperature and improvement in absorption ki-

netics observed in the present study for Mm-oxide catalyzed

MgH2 is more than that reported while using CeO2 alone as a

catalyst [26]. In view of the above, it can be said that the sig-

nificant improvement in hydrogen sorption from MgH2 cata-

lyzed by Mm-oxide is due to synergistic effect [46e48]

produced by the combination of CeO2 and La2O3.

4. Conclusions

Based on the present investigations, the following conclusions

can be drawn:

(i) Mm (Ce and La are the dominant components) and its

oxide exhibits superior catalytic effect for improving the

hydrogen sorption from MgH2. Thus, for the heating rate

of 5 �C/min the onset desorption temperature corre-

sponding to 5 wt.% Mm-oxide catalyzed MgH2 has been

lowered from 381 �C (ball-milled) to 305 �C and that for

MgH2 catalyzed with Mm, it is lowered to 323 �C. Thisdecrease in desorption temperature is due to the catalytic

effect of Mm and Mm-oxide. Thus, the lowering of

desorption temperature for MgH2 when catalyzed by

Mm-oxide and Mm are 76 �C and 58 �C, respectively.(ii) The dehydrogenation activation energy is 101 kJ/mol for

ball-milled, 81 kJ/mol for Mm catalyzed and 66 kJ/mol for

Mm-oxide catalyzed MgH2. The activation energy deter-

mined for the rehydrogenation of ball-milled, Mm and

Mm-oxide catalyzed Mg (dehydrogenated MgH2) in the

present investigations are 91, 70 and 62 kJ/mol.

(iii) The improvement in sorption kinetics of Mm-oxide

catalyzed MgH2 is due to the synergistic effect produced

by the combination of CeO2 and La2O3 in Mm-oxide.

(iv) Based on the above, the cost-effective Mm-oxide ore can

be taken as an effective catalyst for improving hydrogen

sorption from MgH2.

Acknowledgements

Financial support from the Ministry of New and Renewable

Energy (Missionmode project onHydrogen Storage), DST, UGC

and DAE are thankfully acknowledged. Thanks are that due to

Prof. B. Vishwanathan and Prof. S. Srinivasa Murthy for

helpful discussions.

Appendix A. Supplementary data

Supplementary data related to this article can be found, in the

online version at http://dx.doi.org/10.1016/j.ijhydene.2013.04.

040.

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